In a CO2 gas-discharge laser a lasing gas mixture within a laser housing is energized by a radio-frequency (RF) discharge in the gas mixture struck between a pair of parallel spaced-apart electrodes. In a high power CO2 laser, for example, a CO2 slab-laser having an output power of 100 Watts (W) or more, the gas mixture typically includes CO2, nitrogen (N2) and helium He, and is at a pressure between about 50 and 150 Torr. RF voltage (RF power) for driving the laser (energizing the gas-discharge) is provided by the combined output of a plurality of RF amplifiers which are connected to a single RF oscillator, the output of which is optionally pre-amplified. Typically, each of the amplifiers includes two RF-transistor amplifier modules in a push-pull arrangement.
The RF voltage typically required to excite a gas discharge in a CO2 slab laser is about 225 volts at about 80 to 100 MHz drive frequency. Current in the discharge for a constant voltage V applied to increases linearly with power delivered into the discharge. The impedance of the discharge decreases as the RF power into the discharge is increased. A CO2 slab-laser has an efficiency of about 10% for converting RF power into the discharge to laser output power. By way of example, a CO2 laser having 250 W output requires about 2500 W of RF power at a current of about 11 Amps (A) to be delivered into the discharge. The impedance of the discharge is about 20 Ohms.
By way of example, to in order to provide 2500 W of RF power using transistor power modules a minimum of six MOSFET BLF278 modules available from Philips Corporation of Eindhoven, Holland would be required. Outputs of the modules would need to be combined to form a single output that is provided to electrodes generating the laser gas-discharge.
A problem that needs to be addressed in combining the outputs of multiple transistor power amplifier modules is current balancing and phase adjustment of the outputs of each of the individual amplifiers. This is required in order to obtain maximum power output into a load (the discharge electrodes) with minimum back reflection. This back reflection exhibits itself as heat dissipated within the transistor modules.
FIG. 1 schematically illustrates a prior-art arrangement 10 for current and phase balancing RF power amplifiers in a RF combiner type power supply used in driving a CO2 diffusion cooled lasers. Here, the output of a RF oscillator 12 is provided to the input of a driver amplifier 14. The output of the driver amplifier is provided to the input of a 1 to 3 signal splitter (signal divider) 16. Each of three outputs of the splitter is provided to a corresponding power amplifier stage 18. The amplifier stages are nominally identical but are separately designated as stages 18A, 18B, and 18C to reflect the fact that there may be subtle differences due to manufacturing tolerance in the amplifier stages and components thereof. These differences lead to a requirement for the above-discussed current and phase balancing.
In amplifier stage 18A, the corresponding signal from splitter 16 is further split into two portions by a signal splitter (signal divider) D1. One portion is sent to a transistor amplifier module A1 and the other portion is sent to a transistor amplifier module A2. Amplifier modules A1 and A2 are arranged, here, in a push-pull configuration. The amplifier outputs are combined by an impedance matching network Z1. In amplifier stage 18B, the corresponding signal from splitter 16 is further split into two portions by a signal splitter D2. The portions are amplified by transistor amplifier modules A3 and A4, and the amplified outputs are combined by a impedance matching network Z2. In amplifier stage 18C, the corresponding signal from splitter 16 is further split into two portions by a signal splitter D3. The portions are amplified by transistor amplifier modules A5 and A6, and the amplified outputs are combined by a impedance matching network Z3.
The outputs of impedance matching networks Z1, Z2, and Z3 are combined by a RF Output Power Combiner 20. The combined outputs are applied to live electrode 24 of an electrode pair 22 (discharge electrodes) comprising electrode 24 and a grounded electrode 26, spaced apart and parallel to each other. The electrodes are located within a laser housing (not shown) including the lasing gas mixture. An impedance matching network (IMN) 28 matches the output impedance of combiner 20 with the impedance of the discharge electrodes.
Transistor amplifier modules A1-6 are powered by DC voltage from a DC power supply 30. The DC power supply delivers power to each of the transistor amplifier modules A-6 through one of 6 corresponding current sensors CS1-6 respectively. A preferred current sensor is a Hall-effect sensor. A Hall-effect current sensor produces an output voltage in proportion to the current flowing through it. Such a sensor can handle a wide range of currents from sub-amperes to hundreds of amperes in a package compatible with printed circuit board technology.
Current and phase balancing is accomplished by adjusting selectively variable impedance circuits B1-6 connected to a respective input of transistor amplifier modules A1-6. The circuits here are each in the form of a variable shunt (parallel) capacitance. Adjusting the impedances of circuits B1-6 adjusts the input power and phase of inputs into the transistor amplifier modules A1-6, which in turn varies the amount of DC current drawn by the transistor amplifier modules from DC power supply 30. This, in turn again, varies the output power and the phase of the output of transistor amplifier modules, and, correspondingly, varies the amplitude and phase of the RF outputs of impedance matching networks Z1-3.
The impedances of the variable impedance circuits B1-6 are adjusted until the amplitude and phase of each of the inputs to power combiner 20 are equal or nearly equal. The amplitude and phase can be monitored with the aid of an oscilloscope and temporary connections, as in known in the art. The adjustments are necessary to compensate for variations, within manufacturing tolerances, of components of amplifier stages 18A-C. Having the same current and phase out of the output from the amplifier stages is important for achieving maximum RF power delivery into the gas discharge created by electrode-pair 22.
It should be noted here that only sufficient description of apparatus 10 has been provided for understanding the current and phase balancing of the inputs combined by power combiner 20. Detailed descriptions of RF power combiners, and impedance matching networks for the same, are provided in U.S. Pat. No. 7,755,452 and U.S. Pat. No. 7,970,037, each assigned to the assignee of the present invention, and the complete disclosure of each of which is hereby incorporated herein by reference.
While the above-described current and phase balancing method is perfectly adequate for achieving the desired optimization of power transfer to the discharge electrodes in a finished CO2 laser, the method has certain shortcomings from a manufacturing point-of-view, particularly regarding the time required for, and the corresponding cost, of the balancing operation. This time required is relatively long because there is a cross-talk between the amplifiers which makes the balancing operation an iterative process, with a series of re-adjustments required of each variable impedance circuit to converge on a balance point. Considerable skill and experience is required of an operator to master the iterative process. Another shortcoming is the time and cost required for installation current sensors in each of connections between the transistor amplifier modules and the DC power supply. There is a need for a simpler and less time-consuming method current and phase balancing for combined amplifier outputs.